Position sensors are well known for providing feedback control of electromechanical devices such as motors, camera shutters, iris diaphragms, zoom lenses and the like. Variable optical density photo interrupters, referred to herein as gradient wedges, are used with opto-sensors in such positions sensors to detect motion of a member such as a motor shaft or shutter blade to provide an output feedback signal for control purposes. An example of a position sensor and feedback control using an optical gradient wedge is described in commonly assigned U.S. Pat. No. 5,325,142-Depatie et al, the disclosure of which is incorporated herein by reference. Gradient wedges have typically comprised thin metallic films deposited on glass substrates or continuous tone images produced on photographic emulsion coated film stock. Both of these methods produce a wedge that is nonlinear, often non-monotonic and inconsistent from batch to batch. While deposited film wedges can be made accurately, they are generally very expensive to produce and, since they are deposited on glass substrates, they tend to be fragile and have relatively high inertia, a problem particularly in high speed servo control operation. Variable width apertures formed in opaque sheet material have also been described, however these wedges are also costly to produce with reliable repeatability and also suffer from high inertia.
It is therefore desirable to provide position sensors using optical gradient wedges that are easily and economically reproduced and that have good linear response characteristics. It is further desirable to provide such sensors with optical gradient wedges that are accurate and consistent between batches when produced in large quantities and can be produced on thin stock material that does not create high inertia difficulties.